Given Instances

Given instances (or, simply, "givens") define "canonical" values of certain types that serve for synthesizing arguments to context parameters. Example:

trait Ord[T] {
  def compare(x: T, y: T): Int
  extension (x: T) def < (y: T) = compare(x, y) < 0
  extension (x: T) def > (y: T) = compare(x, y) > 0
}

given intOrd as Ord[Int] {
  def compare(x: Int, y: Int) =
    if (x < y) -1 else if (x > y) +1 else 0
}

given listOrd[T](using ord: Ord[T]) as Ord[List[T]] {

  def compare(xs: List[T], ys: List[T]): Int = (xs, ys) match
    case (Nil, Nil) => 0
    case (Nil, _) => -1
    case (_, Nil) => +1
    case (x :: xs1, y :: ys1) =>
      val fst = ord.compare(x, y)
      if (fst != 0) fst else compare(xs1, ys1)
}

This code defines a trait Ord with two given instances. intOrd defines a given for the type Ord[Int] whereas listOrd[T] defines givens for Ord[List[T]] for all types T that come with a given instance for Ord[T] themselves. The using clause in listOrd defines a condition: There must be a given of type Ord[T] for a given of type List[Ord[T]] to exist. Such conditions are expanded by the compiler to context parameters.

Anonymous Givens

The name of a given can be left out. So the definitions of the last section can also be expressed like this:

given Ord[Int] { ... }
given [T](using Ord[T]) as Ord[List[T]] { ... }

If the name of a given is missing, the compiler will synthesize a name from the implemented type(s).

Note The name synthesized by the compiler is chosen to be readable and reasonably concise. For instance, the two instances above would get the names:

given_Ord_Int
given_Ord_List_T

The precise rules for synthesizing names are found here. These rules do not guarantee absence of name conflicts between given instances of types that are "too similar". To avoid conflicts one can use named instances.

Note To ensure robust binary compatibility, publicly available libraries should prefer named instances.

Alias Givens

An alias can be used to define a given instance that is equal to some expression. E.g.:

given global as ExecutionContext = new ForkJoinPool()

This creates a given global of type ExecutionContext that resolves to the right hand side new ForkJoinPool(). The first time global is accessed, a new ForkJoinPool is created, which is then returned for this and all subsequent accesses to global. This operation is thread-safe.

Alias givens can be anonymous as well, e.g.

given Position = enclosingTree.position
given (using config: Config) as Factory = MemoizingFactory(config)

An alias given can have type parameters and context parameters just like any other given, but it can only implement a single type.

Given Macros

Given aliases can have the inline and transparent modifiers. Example:

transparent inline given mkAnnotations[A, T] as Annotations[A, T] = ${
  // code producing a value of a subtype of Annotations
}

Since mkAnnotations is transparent, the type of an application is the type of its right hand side, which can be a proper subtype of the declared result type Annotations[A, T].

Pattern-Bound Given Instances

Given instances can also appear as pattern bound-variables. Example:

for given Context <- applicationContexts do

pair match
  case (ctx as given Context, y) => ...

In the first fragment above, anonymous given instances for class Context are established by enumerating over applicationContexts. In the second fragment, a given Context instance named ctx is established by matching against the first half of the pair selector.

In each case, a pattern-bound given instance consists of given and a type T. The pattern matches exactly the same selectors as the type ascription pattern _: T.

Given Instance Initialization

A given instance without type or context parameters is initialized on-demand, the first time it is accessed. If a given has type or context parameters, a fresh instance is created for each reference.

Syntax

Here is the new syntax for given instances, seen as a delta from the standard context free syntax of Scala 3.

TmplDef           ::=  ...
                   |   ‘given’ GivenDef
GivenDef          ::=  [GivenSig] Type ‘=’ Expr
                   |   [GivenSig] ConstrApps [TemplateBody]
GivenSig          ::=  [id] [DefTypeParamClause] {UsingParamClause} ‘as’